To analyze the amount of oil that is contained in a particular soil and at a particular depth in the proximity of a subterranean well requires extraction of a sample of the well material. An analysis of this material yields the percent of fluid and/or gas contained therein which information is utilized to determine the type of fluid, such as oil, contained therein and the pressure thereof. However, in view of the cost of extracting the core, it is important to extract the core in as intact a condition as possible. Methods for coring a well are discussed in general in U.S. Pat. Nos. 4,312,414 and 4,479,557, issued to Park et al and assigned to Diamond Oil Well Drilling Co.
One factor that can significantly increase the cost per foot of extracted core is jamming of the core during the coring process. Once jammed, the entry of the core into the inner barrel of the coring device is prohibited and the coring device must then be extracted from the bore hole and the jam cleared. However, the presence of a jammed core is difficult to ascertain since the coring process is dependent upon depth measurements at the surface. Therefore, a coring device may have a core jammed therein and the coring procedure continued without knowledge of this jam. This can result in additional damage to the coring device.
One method for preventing this jamming is to monitor the length of the core as it moves up the inner core barrel and compare this with the depth of the drill. A number of devices have been disclosed in U.S. Pat. Nos. 2,555,272, issued to Millison U.S. Pat. No. 3,344,872, issued to Bergan, U.S. Pat. No. 3,605,920 issued to Woodward and U.S. Pat. No. 2,342,253, issued to Cooley. For example, the Millison device utilizes a clockwork instrument disposed in contact with a plug that seals the inner barrel. The clockwork instrument is in contact with the upper end of the inner barrel through a retracting wire. As the instrument is urged upward by the core entering the inner barrel, the retracting mechanism operates numerous gears to record core length information. As a further example, Bergan discloses a device having a chain disposed in the inner barrel from a weight measuring device. As the core moves upward into the inner barrel, the links of the chain are slowly removed, thus reducing the weight of the chain. This weight is measured and data transmitted through a transducer to the surface. Although these prior devices measure the length of the core in the barrel during the coring operation, they do not compensate for the environment at the bottom of the bore hole. During drilling, this environment is subject to high G-forces and pressures. A gear mechanism disposed in the inner barrel with a retracting wire would be such a delicate mechanism that reliability would be questionable.
In view of the above disadvantages, there exists a need for a device for monitoring the movement of the core into the inner barrel in addition to transmitting this information to the surface.